Emulsion polymerized silicone rubber-based impact modifiers, method for making, and blends thereof

ABSTRACT

A method is provided for making emulsion polymerized silicone rubber having an average particle size of from about 400 nm to 2 microns and grafts thereof. Thermoplastic blends having improved weatherability and impact strength are also provided based on the use of a thermoplastic organic polymer and a graft of such emulsion polymerized silicone rubber as an impact modifier.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to silicone rubber-based impactmodifiers in the form of polymerized alkenyl monomer-containing grafts,such as styrene and acrylonitrile, of emulsion polymerized siliconerubber particles having an average particle size in the range of about400 nm or more. More particularly, the present invention relates to theemployment of such polymerized alkenyl containing impact modifiers inthermoplastic resins to provide thermoplastic compositions havingenhanced impact strength and improved weatherability.

[0002] As shown by Craig, U.S. Pat. No. 5,726,270, which is incorporatedherein by reference, aqueous dispersions of organopolysiloxanes areprovided in the form of monomodal organopolysiloxane particles having apre-determined particle size of up to about 2 microns. As discussed inU.S. Pat. No. 5,726,270, current manufacturing practices for makingorganopolysiloxane dispersions often emulsify pre-existingorganopolysiloxane fluids or gums under high shear conditions.Alternative procedures include batch stirring siloxane precursors inwater in the presence of a surfactant. Additional emulsionpolymerization methods are taught, such as shown in U.S. Pat. No.2,891,920, which describes the use of a base catalyst with a cationicsurfactant, and J.P. 62141029 A2 870624, which is directed to acontinuous addition of a pre-emulsion cyclo-siloxane precursor.

[0003] While various procedures are available for making aqueouspolysiloxane emulsions having average particle sizes of about 10 to 300nm, these dispersions often have been restricted to such applications aspersonal care, adhesives and coatings where small particles sizes oftenprovide advantages.

[0004] It would be desirable therefor to be able to make aqueousdispersions of emulsion polymerized silicone rubber particles to expandthe utility of aqueous polysiloxane emulsions to silicone emulsionrubbers having an average particle size of about 400 nm, or greater.

[0005] It also would be desirable to provide low temperature impactmodifiers in the form of polymerized alkenyl monomer-containing graftsof such emulsion polymerized silicone rubber particles.

[0006] In addition, it would be desirable to provide thermoplasticcompositions comprising a thermoplastic polymer and an effective amountof an impact modifier in the form of a polymerized alkenylmonomer-containing graft of an emulsion polymerized silicone rubberhaving an average particle size of 400 nm, or greater.

BRIEF SUMMARY OF THE INVENTION

[0007] The present invention is based on the discovery that siliconerubber particles having a volume average particle size of 400 nm orgreater can be made by a single stage semi-continuous process involvingthe emulsion polymerization of siloxane precursors under low shear,substantially non-homogenizing conditions.

[0008] As used hereinafter, the expression “semi-continuous process”means the introduction under emulsion polymerization conditions ofsilicone rubber siloxane precursors, such asoctamethylcyclotetrasiloxane and γ-mercaptopropyltrimethoxysilane into areactor over an extended period of time, for example, about 2 to about12 hours, and preferably, about 4 to about 8 hours.

[0009] The expression semi-continuous process also includes theemployment of mild, and/or low shear non-homogenizing conditions duringthe emulsion polymerization of the silicone rubber siloxane precursors.The degree of agitation used during the semi-continuous processing ofthe silicone rubber siloxane precursors substantially minimizes theformation of silicone rubber particles having an average particle sizeof below about 400 nm.

[0010] The resulting silicone rubber particles can thereafter berespectively grafted with a polymerizable alkenyl monomer, such as avinyl monomer, to form a polymerized alkenyl polymer shell, to providevaluable impact modifiers for a variety of thermoplastic polymers, suchas polyesters, polycarbonates, polyestercarbonates, polyimides,polyetherimides, and polyamides.

STATEMENT OF THE INVENTION

[0011] There is provided by the present invention, a method for makingan aqueous silicone rubber latex, which comprises,

[0012] (1) semi-continuously adding silicone rubber siloxane precursorsinto a reactor to provide contact under agitation with a reactionmixture comprising water and an acid catalyst-surfactant at atemperature in the range of about 30° C. to about 110° C., and

[0013] (2) recovering a silicone rubber latex comprising silicone rubberparticles having a volume average particle size in the range of about400 nm to about 2 microns.

[0014] There is also provided, a method for making a siliconerubber-based graft copolymer comprising,

[0015] (1) effecting reaction at a temperature of about 50° C. to about95° C. between (A) an aqueous silicone rubber latex having a pH of about3 to about 9, and comprising silicone rubber particles having a volumeaverage particle size in the range of about 400 nm to about 2 microns,and (B) an aqueous mixture comprising at least one polymerizable alkenylorganic monomer, where a sufficient proportion of mixture (B) isutilized in the reaction to provide from about 15% to about 75% byweight of alkenyl polymer shell, based on the total weight of graftcopolymer, and

[0016] (2) coagulating the resulting latex from (1), and (3) recovering,washing, and thereafter drying the resulting solids.

[0017] There is further provided, a method for making a siliconerubber-based graft copolymer comprising,

[0018] (1) effecting reaction at a temperature of about 50° C. to about95° C. between (A) an aqueous silicone rubber latex having a pH of about3 to about 9, and comprising silicone rubber particles having a volumeaverage particle size in the range of about 400 nm to about 2 microns,and (B) an aqueous mixture comprising, styrene and acrylonitrile in aweight ratio of between about 90:10 to about 50:50, where a sufficientproportion of mixture (B) is utilized in the reaction, to provide fromabout 15% to about 75% by weight of alkenyl polymer shell, based on thetotal weight of graft copolymer, and

[0019] (2) coagulating the resulting latex from (1), and

[0020] (3) recovering, washing, and thereafter drying the resultingsolids.

[0021] There is still further provided by the present invention, athermoplastic blend comprising (C), thermoplastic polymer, and (D) about5% to about 50% by weight, based on the weight of thermoplastic blend,of a silicone rubber graft copolymer having about 15% to about 75% byweight of alkenyl polymer shell, and silicone rubber particles having avolume average particle size in the range of about 400 nm to about 2microns.

[0022] Still another aspect of the present invention is directed to amethod for preparing a thermoplastic blend, which comprises mixing (C ),thermoplastic polymer, and (D) about 5% to about 50% by weight, based onthe weight of thermoplastic blend, of a silicone rubber graft copolymerhaving about 15% to about 75% by weight of alkenyl polymer shell, andsilicone rubber particles having a volume average particle size in therange of about 400 nm to about 2 microns.

DETAILED DESCRIPTION OF THE INVENTON

[0023] In the practice of one form of the method of the invention, anemulsion polymerized silicone rubber latex is initially formed bysemi-continuously adding to a reactor containing water, which is beingagitated, such as by stirring, at a temperature in the range of about30° C. to about 110° C., and preferably about 75° C. to about 95° C., amixture of silicone rubber monomers. The semi-continuous addition ofmonomers can be effected, stepwise, and in a dropwise manner, over aperiod of up to about 24 hours. An effective amount of a surfactant canbe used initially in the reactor as part of the agitated aqueousmixture, or it can be introduced with the silicone rubber monomers.

[0024] Among the surfactants which can be used, dodecylbenzenesulfonicacid is preferred. Surfactants which can be used in the practice of theinvention include acid catalyst-surfactants, for example, sulfonicacids, such as alkyl-, and alkaryl-arylsulfonic acids and mixtures ofsurface-active sulfonic acid salts with strong mineral acids. Additionalsulfonic acid catalysts/surfactants are shown in U.S. Pat. No.3,294,725, and Craig, U.S. Pat. No. 5,726,270 which are incorporatedherein by reference.

[0025] Various silicone rubber monomers can be used to form the initialemulsion polymerized silicone rubber latex used in the practice of theinvention. Some of the preferred silicone rubber monomers includecyclosiloxanes, such as octamethylcyclotetrasiloxane, as shown forexample in the Encyclopedia of Polymer Science and Engineering, Volume15, 2nd Edition, pp. 205-308, (1989), John Wiley and Sons. Cross-linkingsilanes include trifunctional such as trimethoxymethylsilane, andtriethoxyphenylsilane, and tetrafunctional, for example,tetraethoxysilane. The cross-linking silanes can be used at from about0.1% to 30% by weight of the silicone rubber monomer mixture. Usefulemulsion polymerizable silicone rubber monomers are for examplecycloalkylsiloxanes, such as hexamethylcyclotrisiloxane, oroctamethylcyclotetrasiloxane which can be copolymerized with from about0.1% to about 30% by weight of a cross-linking agent. Suitablecross-linking agents are for example, tetraalkoxysilane, such as,tetraethoxysilane, and in further combination with analkylacryloxyalkyldialkoxyalkylsilane, as illustrated byγ-methacryloxypropyldimethoxymethylsilane. A comprehensive list ofsilicone rubber monomers can be found in “Silicones”, Hardman andTorkelson, Encyclopedia of Polymer Science and Engineering, volume 15,2nd Edition, pp. 205-308, (1989), John Wiley and Sons, which isincorporated herein by reference.

[0026] In preparing the graft of the emulsion polymerized siliconerubber latex, a suitable polymerizable alkenyl monomer, alone, or incombination as a mixture of two or more alkenyl monomers, such asstyrene, triallyl cyanurate, acrylonitrile, and methylmethacrylate, canbe used in combination with the rubber latex. When a mixture of styreneand acrylonitrile is used, then their weight ratio is between about90:10 to about 50:50.

[0027] The proportion of alkenyl monomer and emulsion polymerized rubberlatex can vary widely by weight. For example, there can be used byweight, from about 15% to about 75% alkenyl monomer, based on the totalweight of graft copolymer.

[0028] In order that those skilled in the art will be better able topractice the invention, the following examples are given by way ofillustration, and not by way of limitation. All parts are by weightunless otherwise indicated.

[0029] Weight percent solids of latex samples are determined afterdrying to a constant weight with a CEM Labwave 9000 gravimetricmicrowave drier. Particle size distributions are obtained using a Nicomp370 Submicron Particle Sizer instrument applying a Gaussian analysisprotocol.

EXAMPLE 1

[0030] There are concurrently added dropwise over a seven hour period,two feed streams into a 2000 ml reactor containing 450 g of water, whichis being agitated continuously and is at a temperature of 86° C. One ofthe feed streams is a solution of 9.35 g of dodecylbenzenesulfonic acidin 300 g of water; the second feed stream is 931 g ofoctamethylcyclotetrasiloxane. The resulting reaction mixture is heatedand agitated for an additional 7 hours. It is then cooled to roomtemperature.

[0031] There is added batch-wise to the above reaction mixture at roomtemperature, 28.3 g of γ-mercaptopropyltrimethoxysilane, and theresulting mixture is reheated to 86° C., and maintained at 86° C. for 12hours. The reaction mixture is allowed to cool to room temperature andcharacterized. There is obtained a silicone rubber latex having 51.5% byweight solids and a volume average particle size of 700 nm.

[0032] A styrene and acrylonitrile mixture is pumped over a two hourperiod into a 5 liter glass reactor which contains an agitated mixtureat 80° C. of 1751 g of distilled water, and 1693 g of the above siliconerubber latex which has been neutralized to a pH of 6.5 with 4 g sodiumbicarbonate dissolved in 50 ml water. The styrene and acrylonitrilemixture consists of 573 g of styrene, 286 g of acrylonitrile, 1.3 g of a75% sodium dioctylsulfosuccinate in 50/50 ethanol/water mixture, and 2.6g t-amylperoxyoctoate. The resulting reaction mixture is heated at 85°C. for an additional two hours. The mixture is then allowed to cool toroom temperature.

[0033] There is obtained a silicone rubber graft copolymer latex. Thesilicone rubber graft copolymer latex has 40.4% total solids by weight,and the volume average particle size is 741 nm.

[0034] The above graft copolymer latex is coagulated in a 1.5% aqueouscalcium chloride solution maintained at 85° C., via slow addition of Ipart of the latex to two parts of calcium chloride solution. Theresulting polymer solids are filtered, washed with distilled water atambient temperatures, and dried in a vacuum oven at room temperature for24 hours, then at 70° C. for at least 24 hours. There is obtained a finepowder.

[0035] The above procedure is repeated to form comparativesilicone-based graft copolymers from comparative silicone rubberlatexes. While substantially the same siloxane monomers are used to makethe comparative silicone rubber latexes, these comparative siliconerubber latexes are not made in a semi-continuous manner in accordancewith the practice of the invention.

[0036] For example, instead of introducing the siloxane reactantsdropwise in a semi-continuous manner, the siloxane reactants areinitially mixed under high shear conditions for 5 minutes at 8000 rpm.The mixture is then passed twice through a homogenizer for 2 passesunder a pressure of 7000 psi to form a stable pre-emulsion. Afterstirring for 6 hours at 80° C., it is allowed to cool to roomtemperature to provide a silicone rubber latex having an averageparticle size of 240 nm. This silicone rubber latex is grafted with astyrene and acrylonitrile following substantially the same procedure asshown above. The resulting silicone rubber graft copolymer latex has 37%of total solids by weight.

[0037] An additional silicone rubber latex is prepared by constantlyagitating a mixture of siloxane precursors for three hours at 89° C.,followed by adding dropwise to the resulting mixture over three hours, apre-emulsion of additional silicone rubber siloxane precursors. Afterpost reacting for three hours, there is formed a silicone rubber latexhaving an average particle size of 170 nm. The resulting silicone rubbergraft copolymer latex has 35% of total solids by weight.

EXAMPLE 2

[0038] Dry blends are prepared from the respective silicone-based graftcopolymers based on respective silicone rubber latexes having an averageparticle size of 170 nm, 240 nm and 700 nm. There are used 54 parts ofthe silicone-based graft copolymer, 46 parts of a 75:25 by weight of astyrene-acrylonitrile copolymer, and I part Irganox 1076 stabilizer. Therespective dry blends are mixed and extruded to obtain pellets using aWelding Engineers 20 mm twin-screw extruder at 450° F. set temperature,400 rpm, and 15-17 lbs/hour throughput. The pellets are injected moldedinto test specimens using an Engel 30 ton injection molder with 450° F.barrel set temperature and 145° F. mold temperature.

[0039] Izod impact values are averages of six samples, specimen width0.125 in, 2 lb pendulum capacity, obtained using a Testing Machines Inc.Monitor/Impact instrument model # 43-02 at room temperature.

[0040] The Izod impact data generated by the thermoplastic materials areshown in the following Table where SAN means styrene-acrylonitrilecopolymer: Graft Copolymer/SAN Silicone Rubber RT Izod Impact (parts byweight) Particle Size (nm) (ft-lb/in) 54/46 240 1.0 54/46 170 1.0 54/46700 3.5

[0041] The above results show that silicone latex made in accordancewith the method of the present invention can provide impact modifierswhich can be used to make thermoplastic blends having enhanced impactstrength and weatherability.

[0042] The impact modifiers of the present invention also can be used toimpart improved impact strength and weatherability to otherthermoplastic blends including those comprising at least one polyester,polycarbonate, polyestercarbonate, polyamide, polyimide, polyetherimide,polyphenylene ether, polystyrene, or a copolymer of styrene withacrylonitrile, methacrylonitrile, esters of acrylic acid, methacrylicacid or copolymers thereof. Such impact modified blends are used in manyapplications requiring good weatherability and low temperature impactstrength, including automotive, building, and construction applications,and in articles of manufacture such as garden furniture, boats, signs,outdoor enclosures for electrical or telecommunications interfacedevices such as smart network interface devices (SNID), and the like.

1. A method for making an aqueous silicone rubber latex, whichcomprises, (1) semi-continuously adding silicone rubber siloxaneprecursors into a reactor to provide contact under agitation with areaction mixture comprising water and an acid catalyst-surfactant at atemperature in the range of about 30° C. to about 110° C., and (2)recovering a silicone rubber latex comprising silicone rubber particleshaving a volume average particle size in the range of about 400 nm toabout 2 microns.
 2. A method in accordance with claim 1, where thesiloxane precursors are added to the reactor over a period of severalhours.
 3. A method in accordance with claim 1, where siloxane precursoris added concurrently into the reactor with water and acidcatalyst-surfactant.
 4. A method in accordance with claim 1, where thesiloxane precursor is a mixture of octamethylcyclotetrasiloxane andgamma-mercaptopropyltrimethoxysilane.
 5. A method in accordance withclaim 1, where the acid-catalyst surfactant is dodecylbenzenesulfonicacid.
 6. A method for making a silicone rubber-based graft copolymercomprising, (1) effecting reaction at a temperature of about 50° C. toabout 95° C. between (A) an aqueous silicone rubber latex having a pH ofabout 3 to about 9, and comprising silicone rubber particles having avolume average particle size in the range of about 400 nm to about 2microns, and (B) an aqueous mixture comprising at least onepolymerizable alkenyl organic monomer, where a sufficient proportion ofmixture (B) is utilized in the reaction to provide from about 15% toabout 75% by weight of alkenyl polymer shell, based on the total weightof graft copolymer, and (2) coagulating the resulting latex from (1),and (3) recovering, washing, and thereafter drying the resulting solids.7. A method for making a silicone rubber-based graft copolymercomprising, (1) effecting reaction at a temperature of about 50° C. toabout 95° C. between (A) an aqueous silicone rubber latex having a pH ofabout 3 to about 9, and comprising silicone rubber particles having avolume average particle size in the range of about 400 nm to about 2microns, and (B) an aqueous mixture comprising, styrene andacrylonitrile in a weight ratio of between about 90:10 to about 50:50,where a sufficient proportion of mixture (B) is utilized in thereaction, to provide from about 15% to about 75% by weight of alkenylpolymer shell, based on the total weight of graft copolymer, and (2)coagulating the resulting latex from (1), and (3) recovering, washing,and thereafter drying the resulting solids.
 8. A method for making asilicone rubber-based graft copolymer in accordance with claim 6, wherethe silicone rubber latex is made by effecting reaction betweenoctamethylcyclotetrasiloxane and gamma-mercaptopropyltrimethoxysilane.9. A silicone rubber-based graft copolymer made in accordance with claim6.
 10. A silicone rubber-based graft copolymer made in accordance withclaim
 7. 11. A thermoplastic blend comprising (C), thermoplasticpolymer, and (D) about 5% to about 50% by weight, based on the weight ofthermoplastic blend, of a silicone rubber graft copolymer having about15% to about 75% by weight of alkenyl polymer shell, and silicone rubberparticles having a volume average particle size in the range of about400 nm to about 2 microns.
 12. A thermoplastic blend in accordance withclaim 11, where the thermoplastic polymer is at least one polyester,polycarbonate, polyestercarbonate, polyamide, polyetherimide,polyphenylene ether, polystyrene, or a copolymer of styrene withacrylonitrile, methacrylonitrile, esters of acrylic acid, methacrylicacid or copolymers thereof.
 13. A thermoplastic blend in accordance withclaim 11, where the thermoplastic polymer is a copolymer of styrene andacrylonitrile.
 14. A thermoplastic blend in accordance with claim 13,where the ratio of styrene to acrylonitrile is in a weight ratio ofbetween about 90:10 to about 50:50.
 15. An article of manufacture madefrom the thermoplastic blend of claim
 12. 16. An article according toclaim 15 comprising an outdoor enclosure for an electrical ortelecommunications interface device.
 17. A method for preparing athermoplastic blend, which comprises mixing (C), thermoplastic polymer,and (D) about 5% to about 50% by weight, based on the weight ofthermoplastic blend, of a silicone rubber graft copolymer having about15% to about 75% by weight of alkenyl polymer shell, and silicone rubberparticles having a volume average particle size in the range of about400 nm to about 2 microns.
 18. A method of claim 17 in which thethermoplastic polymer comprises at least one polyester, polycarbonate,polyestercarbonate, polyamide, polyimide, polyetherimide, polyphenyleneether, polystyrene, or a copolymer of styrene with acrylonitrile,methacrylonitrile, esters of acrylic acid, methacrylic acid orcopolymers thereof.